The document discusses body temperature control and homeostasis. It explains that the hypothalamus acts as the body's thermostat to maintain a constant core temperature near 98.6°F through heat production and heat loss mechanisms. When core temperature increases, the hypothalamus triggers heat loss through sweating and increased blood flow to the skin. When core temperature decreases, it triggers shivering and constricted blood vessels to conserve heat. Factors like exercise, food, and hormones can influence heat production and metabolic rate. A high core temperature kills by denaturing proteins, while a low core temperature causes cardiac issues.

1. Body Temperature Control
Subrato Kumar Barman
Lecturer,
Department of Pharmacy
Ranada Prasad Shaha University
Shitalakhya, Narayanganj-1400.

2. Body Temperature Homeostasis
 Despite wide fluctuations in environmental temperature, homeostatic
mechanisms can maintain a normal range for internal body
temperature.
 If the rate of body heat production equals to the rate of heat loss,
the body maintains a constant core temperature near 37°C (98.6°F).
 Core temperature is the temperature in body structures deep to the
skin and subcutaneous layer.
 Shell temperature is the temperature near the body surface-in the skin
and subcutaneous layer.
 Depending on the environmental temperature, shell temperature is 1–
6°C lower than core temperature.
 A core temperature that is too high kills by denaturing body proteins; a
core temperature that is too low causes cardiac arrhythmias that result
in death.
2

3. Heat Production
 The production of body heat is proportional to metabolic rate. The
overall rate at which metabolic reactions use energy is termed as the
metabolic rate. Some of the energy is used to produce ATP, and some
is released as heat.
 Metabolic rate is measured under standard conditions, with the body in
a quiet, resting, and fasting condition called the basal state. The
measurement obtained under these conditions is the basal metabolic
rate (BMR).
 BMR is 1200-1800 Kcal/Day in adults, or about 24 Kcal/Kg of body
mass in adult males and 22 Kcal/Kg in adult females. The added
calories needed to support daily activities, such as digestion and
walking, range from 500 Kcal to over 3000 Kcal.
3

4. Mechanisms of Heat Transfer
 Maintaining normal body temperature depends on the ability to lose
heat to the environment at the same rate as it is produced by metabolic
reactions.
 Heat can be transferred from the body to its surroundings in four ways:
radiation, conduction, convection, and evaporation.
4

5. Mechanisms of Heat Transfer
1. Radiation: This is the transfer of heat in the form of infrared rays
between a warmer object and a cooler one without physical contact.
 Our body loses heat by radiation than it absorbs from cooler
objects. If surrounding objects are warmer than our body, our body
absorbs more heat than losing by radiation.
 In a room at 21°C (70°F), about 60% of heat loss occurs via
radiation in a resting person.
2. Conduction: This is the heat exchange that occurs between molecules
of two materials that are in direct contact with each other.
 At rest, about 3% of body heat is lost via conduction to solid
materials in contact with the body, such as a chair, clothing, and
jewelry.
 Heat can also be gained via conduction-for example, while soaking
in a hot tub. Because water conducts heat 20 times more effectively
than air, heat loss or heat gain via conduction is much greater when
the body is submerged in cold or hot water.
5

6. Mechanisms of Heat Transfer
3. Convection: This is the transfer of heat by the movement of a fluid (a gas or a
liquid) between areas of different temperature.
 The contact of air or water with our body results in heat transfer by both
conduction and convection.
 When cool air makes contact with the body, it becomes warmed and therefore
less dense and is carried away by convection currents created as the less dense air
rises. The faster the air moves—for example, by a breeze or a fan-the faster the
rate of convection.
 At rest, about 15% of body heat is lost to the air via conduction and convection.
4. Evaporation: This is the conversion of a liquid to a vapor.
 Every milliliter of evaporating water takes with it a great deal of heat about 0.58
Kcal/mL.
 Under typical resting conditions, about 22% of heat loss occurs through
evaporation of about 700 mL of water per day-300 mL in exhaled air and 400
mL from the skin surface. Because we are not normally aware of this water loss
through the skin and mucous membranes of the mouth and respiratory system, it
is termed insensible water loss.
6

7. Hypothalamic Thermostat
 The control center that functions as the body’s thermostat is a group of
neurons in the anterior part of the hypothalamus, the preoptic area.
 This area receives impulses from thermoreceptors in the skin and
mucous membranes and in the hypothalamus.
 Neurons of the preoptic area generate nerve impulses at a higher
frequency when blood temperature increases, and at a lower frequency
when blood temperature decreases.
 Nerve impulses from the preoptic area propagate to two other parts
of the hypothalamus known as the heat-losing center and the heat-
promoting center, which, when stimulated by the preoptic area, set
into operation a series of responses that lower body temperature and
raise body temperature, respectively.
7

8. Thermoregulation
If core temperature
decreases, mechanisms
that help conserve heat
and increase heat
production act via
several negative
feedback loops to raise
the body temperature to
normal.
Figure 1: Negative
feedback mechanisms
that conserve heat and
increase heat production.
8

9. Thermoregulation
If core body temperature rises above normal, a negative feedback loop
opposite to the one depicted in Figure 1 goes into action.
 The higher temperature of the blood stimulates thermoreceptors that send
nerve impulses to the preoptic area, which in turn stimulate the heat-losing
center and inhibit the heat promoting center.
 Nerve impulses from the heat-losing center cause dilation of blood vessels in
the skin. The skin becomes warm, and the excess heat is lost to the
environment via radiation and conduction as an increased volume of blood
flows from the warmer core of the body into the cooler skin.
 At the same time, metabolic rate decreases, and shivering does not occur.
 The high temperature of the blood stimulates sweat glands of the skin via
hypothalamic activation of sympathetic nerves. As the water in perspiration
evaporates from the surface of the skin, the skin is cooled.
All these responses counteract heat-promoting effects and help return body
temperature to normal.
9

10. Factors Affecting Heat Production
Several factors affect the metabolic rate and thus the rate of heat
production:
 Exercise: During exercise, the metabolic rate may increase to 15 times
the basal rate. In well trained athletes, the rate may increase up to 20
times.
 Food intake: The ingestion of food raises the metabolic rate 10-20%
due to the energy “costs” of digesting, absorbing, and storing nutrients.
This effect, food-induced thermogenesis, is greatest after eating a
high-protein meal and is less after eating carbohydrates and lipids.
 Age: The metabolic rate of a child, in relation to its size, is about
double that of an elderly person due to the high rates of reactions
related to growth.
 Body temperature: The higher the body temperature, the higher the
metabolic rate. Each 1°C rise in core temperature increases the rate of
biochemical reactions by about 10%. As a result, metabolic rate may
be increased largely during a fever.
10

11. Factors Affecting Heat Production
 Hormones: Thyroid hormones (thyroxine and triiodothyronine) are
the main regulators of BMR; BMR increases as the blood levels of
thyroid hormones rise. Thyroid hormones increase BMR in part by
stimulating aerobic cellular respiration. As cells use more oxygen to
produce ATP, more heat is released, and body temperature rises. Other
hormones have minor effects on BMR. Testosterone, insulin, and
human growth hormone can increase the metabolic rate by 5–15%.
 Nervous system: During exercise or in a stressful situation, the
sympathetic division of the autonomic nervous system is stimulated. Its
postganglionic neurons release norepinephrine (NE), and it also
stimulates release of the hormones epinephrine and norepinephrine by
the adrenal medulla. Both epinephrine and norepinephrine increase
the metabolic rate of body cells.
 Other factors: Other factors that affect metabolic rate are gender
(lower in females, except during pregnancy aclimate and lactation),
(lower in tropical regions), sleep (lower), and malnutrition (lower).
11

12. Fever
Defintion: Fever is an elevation of core temperature caused by a resetting of the
hypothalamic thermostat.
Causes:
 The most common causes of fever are viral or bacterial infections and
bacterial toxins.
 Other causes are ovulation, excessive secretion of thyroid hormones,
tumors, and reactions to vaccines.
How Fever Develops:
 When phagocytes ingest certain bacteria, they are stimulated to secrete a
pyrogen (pyro-fire; gen-produce),a fever-producing substance.
 One pyrogen is interleukin-1. It circulates to the hypothalamus and induces
neurons of the preoptic area to secrete prostaglandins.
 Some prostaglandins can reset the hypothalamic thermostat at a higher
temperature, and temperature-regulating reflex mechanisms then act to
bring the core body temperature up to this new setting.
Remedy:
 Antipyretics are agents that relieve or reduce fever.
 Examples are acetaminophen (paracetamol), aspirin. All of which reduce
fever by inhibiting synthesis of certain prostaglandins.
12

13. Suggested Reading..
 Tortora, G. J. & Derrickson, B. (2009). Principles of anatomy and
Physiology (12th ed.).USA: John Wiley & Sons.
 Silbernagl, S. & Despopoulos, A. (2003). Color Atlas of Physiology
(6th ed.). Stuttgart, Germany: Thieme.
13